In power conversion applications, a supply voltage signal is necessary for supplying the active elements of a power converter, e.g., the drivers, clock generators, etc. In an AC-DC voltage converter, a tertiary winding is usually adapted to generate a supply voltage signal. FIG. 1 schematically illustrates an AC-DC voltage converter 50 comprising a tertiary winding for providing the supply voltage signal. As shown in FIG. 1, the AC-DC voltage converter 50 receives an AC input signal VAC which is converted to a DC voltage signal VDC of a primary winding of a transformer 51 through a rectifier and a filter capacitor. A main switch 52 is connected between the primary winding of the transformer 51 and a logic ground. The DC voltage signal VDC is further converted to an output voltage signal VOUT of a secondary primary winding of the transformer 51 through controlling the main switch 52 on and off. The transformer 51 further comprises a tertiary winding NT adapted to conduct the DC voltage signal VDC to generate a supply voltage signal VCC for supplying the active elements of the AC-DC voltage converter 50. Although adopting the tertiary winding NT to generate the supply voltage signal VCC of the AC-DC voltage converter 50 is efficient, the tertiary winding NT results in a high cost of the AC-DC voltage converter 50.
Therefore, it is desired to have a solution for efficiently generating the supply voltage signal without the tertiary winding in the AC-DC voltage converter.